Selective cytotoxiec reagents, synthesized by covalently joining cell recognition molecules and toxins, have been reported recently by a number of laboratories. The most successful of these reagents have employed antibodies, as cell recognition molecules, and catalytic toxins as cell killing moieties. The purpose of the research proposed here is to investigate the feasibility of synthesizing cell specific cytotoxic reagents that have therapeutic potential against protozoan and other infectious diseases involving eukaryotic organisms. The organism we will use as a model for infectious parasitic organisms is Acanthamoeba castellanii. Monoclonal antibodies to the cell surface of Acanthamoeba will be produced, and the binding portion of these molecules used as the cell recognition portion of the cytotoxic reagents. Ricin, abrin, diphtheria toxin and various other toxic compounds, as well as hydrolytic enzymes, will be tested for possible use as the toxic portion of the selective cytotoxic reagents. Promising reagents that result from in vitro screening will be employed in vivo toxicity and therapeutic potential experimentation.